As is known, a stage projector extends along a first axis and comprises a first optical system, which has a first focus and is designed substantially to generate and concentrate a light beam in the first focus, and a second optical system set downstream of the first focus and having a second focus, the second optical system comprising a first optical assembly set in a given position along the first axis so that the second focus coincides with the first focus.
A stage projector of the type described above is referred to by the persons skilled in the sector as “wash projector” and is characterized in that it is particularly efficient in terms of brightness. Furthermore, the stage projector of the type described above is characterized in that the outgoing light beam has a cylindrical shape, is homogeneous, and can assume different colourings according to elements that may be inserted and that are selectively arranged in positions in which they intercept either totally or in part the light beam in the first optical system.
The shape and homogeneity of the light beam depend to a large extent upon the fact that the first focus substantially coincides with the second focus. This characteristic, however, presents particular disadvantages in so far as the second focus, i.e., the first optical assembly, must be kept in the given position. In the case where the first optical assembly were not to be kept in the given position, the first and second foci could not be kept in a position of substantial coincidence and the beam would lose in terms of homogeneity. This disadvantage is particularly evident when the light beam is coloured via the elements that may be inserted: when the first and the second foci do not coincide and the light beam is coloured, the light beam generates a spot (projection of light on a surface), which presents a coloured halo and a substantially white patch of light at the centre. Evidently, the need to keep the first and second foci coincident precludes the possibility of having a projector capable of producing a zoom effect.
On the other side documents EP 1,384,941 disclose US 2005/0052872 a stage projector extending along a first axis and comprising: a first optical system, which has a first focus (and is designed substantially to generate a light beam along the first axis and concentrate the light beam in the first focus; and a second optical system, set downstream of the first focus and having a second focus, said second optical system comprising a first optical assembly set in a given position along the first axis so that the second focus coincides with the first focus and a second optical assembly; the first and second optical assemblies being mobile along the first axis in order to modify the form of the light beam and obtain a homogeneous distribution of the light within the light beam.
However, the above “zoom” type projector cannot operate and have the same brightness of a “wash projector”.